Evolution of the Disk Gap Band Parachutes for US Mars missions
From the first Viking lander to the last Mars Rover the US robotic Martians have always descended safely under a Disk-Gap-Band parachute. The Disk Gap Band parachute is one of the oldest and simplest forms of the parachute. In the constructed area it is quite literally a disk, a gap and a band. Originally designed for high-altitude Meteorological sounding rockets it was one of the three parachute types tested in the NASA PEPP program.
This program, the Planetary Entry Parachute Program and the follow-up SPED led to the selection of the parachute for the Viking Mars mission. The other parachutes in the race were a Ringsail and cross parachute. Besides being relatively simple to produce a Disk Gap Band parachute has the advantage that it has a low packing volume and operates very well at high altitudes. The PEPP, SPED, and BLDT programs were very complicated and expensive but gathered a huge wealth of data. This led to the selection of the DGB for all future US Mars missions.
Early Disk Gap Band parachutes came from the need of tracking small objects high in the atmosphere. Early Meteorological sounding rockets worked by launching a payload with parachutes to high altitudes and following the descent of the payload. Based on this descent scientists could determine the air density and wind speeds at altitudes. Clint Eckstrom found that this need could be very well solved by the Mylar material, a material by DuPont, used on the ECHO communication satellites for its great reflective purpose. Because Mylar is impermeable, it does not allow any air to pass, Eckstrom added a vent hole and a gap for stability.
When in 1966 NASA began their race to land on Mars, there was a new need for a parachute decelerator that could work in supersonic conditions and at a low density. To identify the quality of various parachutes under these conditions the PEPP program was started. For PEPP a balloon with a rocket as a second stage was launched from White Sands. When the balloon would reach cruising altitude, the rocket motor would boost the test vehicle to the right conditions. Other PEPP flights were done using a pure-sounding rocket solution. Tests were done with various parachute types and sizes. For the Disk Gap Band parachutes between 9.1 and 19.7 meters were tested at Mach numbers ranging from 1.56 to 2.72. The shape of the Disk Gap Band parachutes used for PEPP was largely similar to those designed and created by Eckstrom, but they did incorporate some structural improvements. The initial knowledge of PEPP led to the DGB being selected for the Mars Voyager mission. When in 1967 Mars Voyager was cancelled and replaced by Mars Viking the testing of these parachutes continued under the SHAPE program.
Disk Gap Band parachute being tested during BLDT
For the SHAPE program, the search was narrowed to a Disk Gap Band parachute of 12.2 meters and tests were done at Mach numbers between 2.58 and 3.31. It is worth noticing that the test done at Mach 3.31 caused massive damage to the canopy marking the first time a large-scale DGB was damaged during a supersonic flight. The other SHAPE flights included testing of a reefed DGB parachute. This failed at first (SHAPE flight 2) but was later performed successfully in the third SHAPE flight. A SHAPE-II program consisting of a single flight was performed to analyze the behaviour of a Disk Gap Band behind a blunt body.
When the Disk Gap Band parachute was then selected for the Viking mission it was tested extensively in subsonic, transonic and supersonic wind tunnels. After the wind tunnel phase, a drop test phase was done using a B-57 bomber. When the design entered the final phases the Balloon-Launched-Decelerator Test program (BLDT) was started. These tests were similar to earlier PEPP and SPED tests but this time as qualification testing for the Viking Mars lander.
A similar Disk Gap Band was later used for the Pathfinder mission. The parachute was however scaled down from 16.15 meters to 12.74 meters. This parachute was the first one to use high-tenacity fibres such as Kevlar as the primary structural components. These fibres tend to be lighter and stronger than nylon or Dacron but much stiffer. From this flight onwards the Disk Gap Band parachute would find its way into all Mars landers with some slight changes. The design was better understood and improved, but the basic design principles would remain the same.
Disk Gap Band for Perseverance tested during in a wind tunnel
